I am replying with regards to your last reply – what Maryanne says may be correct but you have implemented an excellent idea which has worked and put measures in place to see if they are cheating.
What you need to do now is see if by increasing calls per hour is see if the following have increased:
* Repeat calls
* Short calls (less than 10secs)
* Has quality been compromised
* Outgoing calls including transfers
If any of the above has increased I would take measures to reduce their bonuses.
There are always people who will work the system to get money so be careful